Illumination condenser for a particle optical projection system

CROSS-REFERENCE TO RELATED APPLICATIONS

p-0002This application claims priority of International Application No. PCT/EP2005/050941, filed Mar. 3, 2005 and German Application No. 10 2004 019 835.7, filed Apr. 23, 2004, the complete disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

p-0003a) Field of the Invention

p-0004The invention is directed to an illumination condenser for a particle optics projection system.

p-0005b) Description of the Related Art

p-0006In semiconductor fabrication, it is often necessary to illuminate an object (mask or multi-beam modulator) telecentrically by means of a condenser. In so doing, the partial crossover belonging to an off-axis point of the mask is usually directed away from the axis due to the spherical aberration of the condenser, which results in additional errors (inclined beam incidence in target, astigmatism and/or distortion). The off-axis crossover causes additional aberrations in the particle optics projection device because the beams pass the lens fields at a greater distance from the axis. It is essentially the spherical aberration of the illumination condenser that causes the partial crossover to be off-axis. The effect of the spherical aberration of the illumination condenser is proportional to the third power of the operative aperture in the crossover. In large-field particle optics projection systems, this aperture is very large, typically 100 mrad. It can be reduced in existing illumination condensers only by a very large distance between the particle source and the mask or multi-aperture multi-beam modulator. However, this results in an excessive structural height of the particle beam projection system.

p-0007U.S. Pat. No. 5,742,062 describes an electrostatic lens which comprises a plurality of annular electrodes to which different potentials are applied. This lens is suited to enable a large-field parallel illumination of an object (mask) with charged particles. It is possible to minimize lens aberrations through suitable selection of the electrode potentials. It is disadvantageous that elaborate technical resources are required to provide a plurality of different potentials in a highly consistent manner over time. In case of high beam energy (50 . . . 200 keV), long insulation distances are required, the vacuum feedthroughs are correspondingly large, and therefore the required vacuum volumes for this arrangement are also large. This leads to the disadvantage that the shielding of the particle beam from magnetic interference fields is very extensive because suitable shielding cylinders, e.g., made of Mu metal, require a large radius and thick walls. Further, the outgassing of the large surface of the electrodes and insulators is disadvantageous for the desired low final pressure in the chamber and for a suitably low contamination rate.

p-0008Rose's paper on the magnetic-electrostatic multielectrode lens (D. Preikszas, H. Rose, Optik 100 (1995), 179) discloses a combined magnetic-electrostatic lens comprising a quantity of identical elements (apertures, ring currents) which are controlled in such a way that aberrations are suppressed to a great extent.

p-0009The prior art is characterized in that the spherical aberration of the illumination condenser in particle beam projection devices is reduced, if at all, only over the large radial extension of the condenser. This substantially limits the possible size of the mask and/or of the multi-aperture beam modulator. There have only been attempts with electrostatic condenser lenses having very many electrodes (multi-aperture condenser) to achieve a shape of the electrostatic field of the condenser such that the spherical aberration is extensively suppressed. However, an electrostatic multi-aperture condenser of this kind requires a large quantity of high-voltage supplies for the individual electrodes and is therefore only usable to a limited extent.

OBJECT AND SUMMARY OF THE INVENTION

p-0010It is the primary object of the invention to develop a projection device for exposing substrates such that a mask or a multi-aperture beam modulator is illuminated preferably in a telecentric manner. Further, the structural height of the projection device should be kept within reasonable limits.

p-0011It is advantageous when the illumination condenser for a particle optics projection system is formed of a magnetic lens and when the magnetic lens comprises a plurality of gaps. The quantity of gaps is greater than or equal to five. The magnetic lens comprises a sequence of a plurality of partial lenses. Each of the plurality of partial lenses comprises a separate winding, each winding being controllable individually. At least two adjacent partial lenses have a common pole piece.

p-0012Further, it is advantageous when the illumination condenser is constructed in such a way that the aberration of the source crossover is minimized, wherein the radial extension, the maximum bore hole radius R_kof the illumination condenser, is less than 150 mm.

p-0013Further, it can be advantageous that an illumination condenser of this kind is combined with an electrostatic diverging lens which further minimizes aberrations.

p-0014Further, it can be advantageous that an illumination condenser of this kind is constructed as a multiple-gap doublet (two multiple-gap lenses one behind the other with opposed image rotation). In this case, anisotropic residual errors would be compensated.

p-0015This object is met by a system for a particle optics projection system comprising an illumination condenser which is formed of a magnetic lens and a magnetic lens comprising a plurality of gaps.

p-0016It is particularly advantageous when the illumination of the mask is carried out in such a way that the partial crossover of a point on the mask or multi-aperture beam modulator remains on the optical axis of the particle beam projection device as far as possible.

p-0017The subject matter of the invention is shown schematically in the drawings and is described in the following with reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0018<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic view of the beam path in a particle optics projection system;

p-0019<figref idrefs="DRAWINGS">FIG. 2</figref> shows an embodiment example of a magnetic illumination condenser according to the invention; and

p-0020<figref idrefs="DRAWINGS">FIG. 3</figref> shows the axial field distribution of the magnetic illumination condenser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

p-0021<figref idrefs="DRAWINGS">FIG. 1</figref> shows the basic beam path in a particle optics projection system <b>2</b>. The following description is directed to the particle optics projection system <b>2</b>, wherein the particles are electrons. An electron beam <b>31</b> is generated by an electron gun (particle gun) <b>30</b> and propagates in direction of an electron-optical axis <b>32</b>. The electrons exiting from the electron cannon <b>30</b> have a source crossover <b>31</b>₀. A beam centering device <b>33</b> which orients the electron beam <b>31</b> symmetrically around the optical axis <b>32</b> is arranged downstream of the electron cannon <b>30</b>. After the beam centering device, the electron beam <b>31</b> traverses an illumination condenser <b>10</b> which forms a parallel beam from the initially divergent electron beam <b>31</b>. The beam formed by the illumination condenser <b>10</b> has a diameter over which the intensity is homogeneously distributed. An aperture <b>34</b> with a plurality of openings <b>35</b> for the electron beam is provided downstream of the illumination condenser <b>10</b>. After the deflecting plate <b>34</b> there follows an acceleration lens <b>36</b> followed by at least one magnetic lens <b>50</b> for beam rotation. Two additional magnetic lenses <b>37</b> which serve to reduce the imaging of the aperture plate <b>34</b> are shown in the present embodiment example. Before the electron beam <b>31</b> strikes the target <b>40</b>, e.g., a wafer, the electron beam <b>31</b> passes through an objective lens <b>38</b>. The objective lens <b>38</b> is outfitted with a plurality of elements. A deflecting device <b>42</b> is provided before and after a second crossover <b>31</b>₂of the electron beam <b>31</b>. The deflecting device <b>42</b> serves for deflecting and for determining the position of the electron beam <b>31</b> or the plurality of individual beams generated through the aperture plate <b>34</b>. Further, a focusing lens <b>44</b> is provided for dynamic focusing, and a coil arrangement <b>46</b> is provided as a stigmator. Across from the target <b>40</b>, the objective lens <b>38</b> has a height sensor <b>48</b> and a detector for the electrons which are backscattered by the target <b>40</b>.

p-0022<figref idrefs="DRAWINGS">FIG. 2</figref> shows an embodiment form of an illumination condenser <b>10</b> according to the invention. The illumination condenser <b>10</b> comprises a magnetic lens <b>11</b> formed of a plurality of gaps <b>12</b>. A gap number ny>=5 is preferred. The illumination condenser <b>10</b> presents a magnetic lens with a long focal length. The focal length is typically around 500 mm. The particle beam exiting from the illumination condenser <b>10</b> illuminates a mask or a multi-aperture beam modulator or an aperture plate <b>40</b>, preferably telecentrically. For this purpose, the source crossover <b>31</b>₀must lie in the object-side focal point of the illumination condenser <b>10</b>, i.e., the source crossover <b>31</b>₀is imaged at infinity.

p-0023In the embodiment example of a magnetic illumination condenser <b>10</b> according to the invention shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, the magnetic lens <b>11</b> comprises five gaps. The illumination condenser <b>10</b> consequently comprises a sequence of a plurality of parallel lenses <b>14</b>, <b>15</b>, <b>16</b>, <b>17</b> and <b>18</b> which are arranged in the beam path <b>20</b> of the particle optics projection device <b>2</b> in front of the mask or an aperture plate <b>40</b> and illuminate the latter preferably telecentrically. Each partial lens <b>14</b>, <b>15</b>, <b>16</b>, <b>17</b>, <b>18</b> has a first winding <b>14</b><i>a</i>, <b>15</b><i>a</i>, <b>16</b><i>a</i>, <b>17</b><i>a</i>, <b>18</b><i>a </i>with separately adjustable excitation. The adjustment of the separate excitations of the partial lenses can also be carried out by means of an individual number of turns for the first windings <b>14</b><i>a</i>, <b>15</b><i>a</i>, <b>16</b><i>a</i>, <b>17</b><i>a</i>, <b>18</b><i>a</i>. The windings which are operated in series can then be supplied from a common power source. Further, it may be advantageous to provide second windings (fine windings) <b>14</b><i>b</i>, <b>15</b><i>b</i>, <b>16</b><i>b</i>, <b>17</b><i>b</i>, <b>18</b><i>b </i>in addition to the first windings (coarse windings) <b>14</b><i>a</i>, <b>15</b><i>a</i>, <b>16</b><i>a</i>, <b>17</b><i>a</i>, <b>18</b><i>a</i>. The second windings (fine windings) <b>14</b><i>b</i>, <b>15</b><i>b</i>, <b>16</b><i>b</i>, <b>17</b><i>b</i>, <b>18</b><i>b </i>preferably have individual numbers of turns for each partial lens <b>14</b>, <b>15</b>, <b>16</b>, <b>17</b>, <b>18</b>. By varying the excitation of the series-connected fine windings, a sensitive focal length variation can be carried out in a simple manner when the series connection of the coarse windings is carried out from a shared constant-current source at the same time. Two adjacent partial lenses share a common pole piece <b>19</b>. The excitations and gaps of the partial lenses <b>14</b>, <b>15</b>, <b>16</b>, <b>17</b>, <b>18</b> and the diameter of the pole piece <b>19</b> differ from one another.

p-0024In the embodiment example with five gaps shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, a total of sixteen parameters can be selected individually in such a way that the spherical aberration of the illumination condenser <b>10</b> in the imaging of the source crossover <b>31</b>₀is minimized, provided that the radial extension, the maximum bore hole radius R_kof the illumination condenser <b>10</b>, does not exceed a certain size. A typical size for the bore hole radius R_kof the illumination condenser <b>10</b> is less than 150 mm.

p-0025Only one half of the illumination condenser <b>10</b> is shown in <figref idrefs="DRAWINGS">FIG. 2</figref>. The illumination condenser <b>10</b> is rotationally symmetric around an axis <b>21</b> of the beam path <b>20</b>. The beam path <b>20</b> passes through the illumination condenser <b>10</b> from a beam entrance A to a beam exit B. In the illumination condenser <b>10</b> of the present invention, the magnetic flux lines <b>22</b> develop in such a way that they recede farther from the axis <b>21</b> of the beam path <b>20</b> as point B of the illumination condenser <b>10</b> is approached. In other words, the magnetic field strength <b>24</b> represented by the magnetic flux lines <b>22</b> is smaller at the beam exit A of the illumination condenser <b>10</b> than at the beam entrance B.

p-0026<figref idrefs="DRAWINGS">FIG. 3</figref> shows the curve of the axial field distribution of an optimized illumination condenser <b>10</b>. Since the source crossover <b>31</b>₀is preferably imaged at infinity, the illumination condenser <b>10</b> is optimized in such a way that the quotient of the circle of confusion of the crossover and the diameter of the crossover is minimal when the crossover approaches infinity in the limiting case. The magnetic field strength <b>24</b> of the illumination condenser <b>10</b> according to the invention is shown as a function of the position with reference to the axis <b>21</b> of the beam path. It can be seen that the magnetic field strength <b>24</b> in the area of point A increases sharply and reaches a maximum at a short distance. Proceeding from the maximum, the magnetic field strength <b>24</b> then steadily decreases and reaches zero approximately at point B.

p-0027The invention has been described with reference to a particular embodiment example, but it will be self-evident for the person skilled in the art that changes and modifications may be carried out within the framework of the technical expertise of the person skilled in the art without departing from the protective scope of the appended patent claims.